A coin is generally
a piece of hard material, traditionally metal
and usually in the shape of a disc, which is used
as a form of money. With banknotes, coins make
up the cash forms of all modern money systems.
Coins are used for lower valued units, notes for
the higher values.
Coin collecting is the hobby of collecting coins.
Coins have been considered collectable objects,
for various reasons, for at least the past several
thousand years.

Coin collectors
often maintain fairly specialised interests. Examples
include coins of a certain nation or historic
period, world coins, and error coins, as well
as exonumia. It is common for collectors of national
coins to specialise in the coins of their own
country. Popular ways to collect national coins
include collecting one of every date and mint
mark for a particular series (date/mint mark sets),
and collecting a representative coin of each different
series (type sets).

Collectors of
ancient and medieval coins are usually more interested
in historical significance than other collectors.
Coins of Roman, Byzantine, Greek, Celtic, Parthian,
Merovingian, Ostrogothic and ancient Israelite
origin are amongst the more popular coins of this
type. Specialties tend to vary greatly, but one
prevalent approach is the collection of coins
minted during a particular emperor's reign.

Collectors of
world coins are often interested in geography.
They can "travel the world" vicariously
through their collecting. A popular way to collect
world coins is to acquire representative examples
from every country or coin issuing authority.

The collecting
of error coins is a modern development, made possible
through the automation of coin manufacturing processes
during the 19th century. Collectors of ancient
and medieval coins accept coin "errors"
because manual coin manufacturing proceses lend
unique features to each coin struck. Collectors
of modern coins find errors desirable because
modern processes make the likelihood of their
production extremely limited. Types of coin errors
include double strikes, off metal coins, displaced
or off center coins, clipped coins, and mules
(different denominations on two sides of one coin).

Several coin
grading services will grade and "encapsulate"
coins in a labeled, air-tight plastic holder.
Two highly respected grading services are the
Numismatic Guarantee Corporation (NGC) and Professional
Coin Grading Service (PCGS). However, professional
grading services are the subject of controversy
because grading is subjective—a coin may
receive a different grade by a different service,
or even upon resubmission to the same service.
Due to potentially large differences in value
over slight differences in a coin's condition,
some commercial coin dealers will repeatedly resubmit
a coin to a grading service in the hopes of a
higher grade.

The first international
convention for coin collectors was held in August
15–18, 1962, in Detroit, Michigan, sponsored
by the American Numismatic Association and the
Canadian Numismatic Association. Attendance is
estimated at 40,000. The scientific study of coins
is known as "numismatics".